In integrated circuits, such as microprocessors, memories, and the like, signals may be routed for relatively long distances using transmission lines. A transmission line may be a bus, a printed circuit board trace, or other type of relatively long metal line for transporting a digital signal. Typically, a printed circuit board trace has a characteristic impedance of between 50 and 75 ohms. In CMOS (complementary metal-oxide semiconductor) circuits, the input impedance of a gate of a CMOS transistor is usually very high. The receiving end, or far end, of the transmission line is typically connected to an input of a logic circuit, where the input impedance is higher than the characteristic impedance of the transmission line. If the impedance coupled to the far end of the transmission line is different than the impedance of the near end, or sending end impedance, the signal may be reflected back to the sending end, causing the signal to overshoot a planned steady state voltage for the logic state. The signal may be reflected back and forth many times between the near end and the far end, causing oscillatory behavior of the signal at both ends. This repeated overshooting and undershooting of the signal is commonly known as "ringing", and results in reduced noise immunity and increased time for the signal to become, and remain, valid at the far end. Impedance matching is the practice of matching the impedance of the driver and/or the load to the characteristic impedance of the transmission line to facilitate the most efficient transfer of power.
A driver circuit is used to provide enough current to "drive" a signal the length of the transmission line. A CMOS driver circuit commonly includes a P-channel transistor and an N-channel transistor connected in series between a positive power supply voltage terminal and a ground terminal. The gates of the transistors receive an input signal, and an output terminal of the driver circuit is located between the transistors. The P-channel transistor functions as a "pull-up" transistor, and the N-channel transistor functions as a "pull-down" transistor. The output impedance of the driver circuit should match the characteristic impedance of the transmission line in order for the driver circuit to absorb the reflected signal and prevent ringing. This would result in quieter waveforms, better noise immunity, and improved signal timing margin.
When driving a digital signal, it is desirable for the signal to transition from one logic state to another logic state very quickly. To increase the driving capability of a CMOS driver, the size of the P-channel and N-channel transistors are increased. However, increasing the size of the transistors also lowers the resistance of the transistors. A CMOS driver with an output impedance of 50-75 ohms to match the transmission line characteristic impedance would be too weak to switch the transmission line fast enough, for most reasonably sized transmission lines.